Technical Field
The present disclosure relates to a toner and an image forming method.
Description of the Related Art
In recent years, toner is required to be fixable at low temperatures in electrophotography. This requirement has arisen from the demand for saving energy by reducing energy required for fixing image, and also from the demand for electrophotographic image forming apparatus having a higher processing speed and smaller size, and outputting an image of higher image quality.
Generally, the image quality deteriorates as the speed is raised and the size is reduced. There are various reasons for this phenomenon. Among them, a fixing failure that may occur in the fixing process is a great cause.
In the fixing process, an unfixed toner image is fixed on a recording medium, such as a paper sheet, by heat and pressure. As the system speed becomes higher, the unfixed toner image cannot receive a sufficient amount of heat in the fixing process. As a result, a fixing failure occurs, causing a rough surface of the fixed toner image or a defective image containing a residual image called cold offset. In order not to degrade image quality by raising the system speed, the fixing temperature can be raised. However, raising the fixing temperature is not the best measure, because heat leaking from the fixing device adversely affects other processes in the image forming apparatus, the wear speed of the fixing member is accelerated, and the amount of consumed energy is increased.
In particular, in high-speed image forming apparatus, toner itself is required to improve fixing performance. More specifically, toner fixable at much lower temperatures is demanded.
Various attempts have been made so far for improving fixability of toner. For example, controlling thermal properties (e.g., glass transition temperature (Tg) and softening temperature (T1/2)) of the binder resin of toner is known as one method for improving fixability of toner. On the other hand, lowering of Tg of the resin may deteriorate heat-resistant storage stability of the toner, and lowering of T1/2 by lowering the molecular weight of the resin may cause a problem such as hot offset. Toner satisfying all of low-temperature fixability, heat-resistant storage stability, and hot offset resistance has never been obtained by simply controlling thermal properties of the resin.